The subject matter disclosed herein relates to turbine blade tip wear, and more particularly, a cooled turbine blade including a coating system and methods for eliminating turbine blade tip wear in conjunction with a CMC or metal shroud in a gas turbine engine.
The turbine section of a gas turbine engine contains a rotor shaft and one or more turbine stages, each having a turbine disk (or rotor) mounted or otherwise carried by the shaft and turbine blades mounted to and radially extending from the periphery of the disk. A turbine assembly typically generates rotating shaft power by expanding hot compressed gas produced by combustion of a fuel. Gas turbine buckets or blades generally have an airfoil shape designed to convert the thermal and kinetic energy of the flow path gases into mechanical rotation of the rotor.
Within a turbine engine, a shroud is a ring of material surrounding the rotating blades. Ceramic matrix composites (CMCs) are an attractive material for turbine applications, particularly shrouds, because CMCs having high temperature capability and are light weight. However, CMC components must be protected with an environmental barrier coating (EBC) in turbine engine environments to avoid oxidation and recession in the presence of high temperature air flow. Alternatively, metal components may be protected with a thermal barrier coating (TBC) to avoid oxidation and recession in the presence of high temperature air flow.
Turbine performance and efficiency may be enhanced by reducing the space between the tip of the rotating blade and the stationary shroud to limit the flow of air over or around the tip of the blade that would otherwise bypass the blade. For example, a blade may be configured so that its tip fits close to the shroud during engine operation. Thus, generating and maintaining a small tip clearance is particularly desired for efficiency purposes.
During engine operation, the blade tips can sometimes rub against the shroud, thereby increasing the gap and resulting in a loss of efficiency, or in some cases, damaging or destroying the blade set.
To reduce the loss of efficiency, an abradable layer may be deposited on the top of the EBC or TBC on the shroud, or the EBC (or TBC) may serve as the abradable layer. In the high environmental temperatures found in an aircraft engine and a gas turbine, the metal blade strength is decreased and the blade-shroud rubbing further elevates the temperature of the blade tip due to friction caused thermal effect, resulting in severe blade wear. Accordingly, the abradable layer is required to be “softer” than the blades at working temperatures. The abradable layer however cannot be too soft, as they will be eroded too fast. In one particular instance, the abradable layer is formed as, a continuous ceramic layer and is typically quite hard. The hardness of this continuous abradable layer may cause it to not abrade, but rather will cause the tips of the rotating blades to abrade.
In another particular instance, the abradable layer is formed as a series of ceramic ridges that break away upon contact with the rotating blade tip. The ceramic material is typically made out of the same ceramic material as one of the environmental barrier layers, for example, rare earth disilicate or barium strontium aluminosilicate (BSAS). Current efforts in developing abradable materials for gas turbines rely on patterned (camberline, straight line, diamond) or flat (dense and porous) ceramic coatings for the EBC coated shroud while maintaining a reasonable erosion resistance. However, the patterned ridges on the surface of the shroud reduce aerodynamic efficiency and tend to be more expensive and have less thermal protection.
Thus, an improved design of a turbine system using a metal blade and an EBC coated CMC component, particularly a shroud, is desirable in the art.